Checkerwork for multizone regenerators



July 3, 1934. H. Pol-u. ET AL CHECKERWORK FOR MULTIZONE REGENERATORS y Filed Sept. Il, 1931 Patented July 3, 1934 UNITED STATES CHECKERWORK FOR MULTIZONE REGENERATORS Hans Pohl, Bad Tonnistein, and Alfred Schack, Dusseldorf, Germany Application September 11, 1931, Serial No. 562,346 In Germany September 25, 1930 1 Claim.

This invention relates to improvements in blast furnace air heaters, more especially multi-zone Cowper regenerators. i

The most satisfactory checkerwork for regenerators hitherto known isthat of the so-called multizone regenerator. This is characterized by the fact that in the hottest layer thick bricks are used having passages 6 to 8 inches wide which gradually lead over to a fine-meshed checkerwork l having passages down to one inch in width, the' thiclmess of the walls being likewise gradually reduced. This arrangement is chosen for the following reasons:-

Wide passages are provided in the hot zone in l order to obtain a strong heat-radiation of the gas currents whereby the coeicient of heat transfer is considerably increased during the heating or gas period. Moreover wide passages are'less subjected to softening. 'Ihis kin'd of checker work 2o underlies to the following conditions of temperature:-

The wide passages in the hot zone result in a relatively strong radiation and a relatively low speed of the gas currents. As taught by recent searches, a combustion gas containing carbon dioxide and water vapor delivers great amounts of heat by radiation at temperatures above 500 C. This radiation increases with increasing thickness of the layer of the radiating gas, and above all with increasing temperature. In addition to this heat transfer by radiation a further amount of heat is delivered from the gas by contact with. the surface to be heated (delivery by convection). In contradistinctionv to the heat transfer by radiation, the heat transfer by convection decreases with increasing thickness of the gas layer, i. e. with increasing diameter of the passages and moreover depends upon the speed of the gas in such a way that the convection decreases and increases with the speed of the gas. Now as the above-described checkerwork, besides the great width of the single passages, has a great free total cross-section, the streaming velocity of the gas and therewith the heat transfer by convection is relatively small. Therefore the usual arrangement of wide passages in the hot layers results in a large heat transfer by radiation, and a small heat transfer by convection. As the heat transfer by radiation at the temperatures of l000 C. and

more have coming into consideration is by far superior to that by convection, the total heat transfer during the gas period is a considerable one with the usual widemeshed checkerwork.

Quite different are the conditions appearing during the blast period. As the air does not contain appreciable amounts of carbon dioxide and Water Vapor, the gas radiation does not take a noticeable part in the transfer of heat. Consequently the air current during the blast period of the regenerator absorbs heat by convection only and, as the absorption of heat by convection, as above mentioned, decreases with the increasing diameter of the passages and decreases with decreasing velocity, the heat transfer during the blast period is small in the hot part of the regenerator. Strictly speaking not the transfer of heat or the passage of heat is concerned, but the coeiilcient of heat transfer, i. e. the heat transfer in calories per square meter and hour and per centigrade of difference between the temperatures. Assuming for the sake of simplicity a plant comprising two regenerators, of course the total heat transfer during the gas period per square` meter of heating surface must be equal to the total heat transfer during the blast period per square meter of heating surface. 'Ihe above explained differences in the heat transfer then manifest themselves in that the difference between the temperatures of the heating gas andthe surface to be heated is small owing to the high coefficient of heat transfer and the difference between the temperatures of the heating surface and the blast is great owing to the small coefficient of heat transfer. These conditions of heat transfer practically mean that the average temperature of the bricks in the described regenerators approaches the temperature of the gas and therefore is very high, whereas the temperature of the air remains relatively low. (See, for instance, equation No. 428 in the text-book by A. Schack Der industrielle 9o Wrmebergang, Dsseldorf, editor Stahl- Eisen.) This high brick temperature is dangerous and often results in destruction of the regenerator. Having recognized these relations the inven- 05 tors do away with such inconveniences by employing even in the hot zone narrow passages having a diameter of four inches or less. VHereby the rate of flow and therewith the coeiicient of heat transfer by convection during the gas and bl'ast periods are increased. Simultaneously the radiation during the gas period decreases to such an extent, that all in all the coeiiicient of total heat transfer during the gas period is decreased and the coefcient of total heat transfer during the blast period is considerably increased. The coefficients of heat transfer are in this manner approached to one another. As a rule, we have found that the passages in the hot zone should have a free crosssectional area aggregating less than the narrower passages in the colder zones and only 40 per cent. or less of the total cross-sectional area.

By reducing the width of the passages in the hot zone in the manner herein described it is forthwith possible to reduce the temperature of the bricks as compared with the corresponding zone of a usual multizone regenerator which in all other respects is worked under the same conditions.

Besides the temperatures the coeicient of heat transit lc (see equation 427 in the abovecited book by Schack) is decisive for the total heat transfer. -This coefficient lc is composed of the two coefficients of heat transfer during the gas and blast period. It is always smaller than the smallest of the coefficients of heat transfer contained therein. Therefore with the multizone regenerators hitherto known the coecient of heat transit in the hot layers is small, although, as above described, the coefficient of heat transfer is great during the gas period; for the coeicient of heat transfer during the blast period is small in this regenerator and therefore the coefllcient of heat transit is still smaller. As according to this invention the coeicient of heat transfer during the blast period is considerably increased, it results that although the coefficient of heat transfer of the gas is decreased the coelcient of heat transit and therewith the total heat transfer are increased by 50 per cent. and more. Accordingly a second advantage of our invention consists in the increase of the heat transit per square meter of heating surface in the hottest parts. Furthermore according to this invention the heating surface per cubic meter of the space for containing the checkerwork is increased. Finally the stability of the checkerwork as compared with the old wide-meshed checkerwork is increased.

As to-day in any case puried waste gas from the blast furnace is employed, there are no objections to the reduction in diameter of the passages.

Accordingly the present invention consists in reducing the size of the passages lin the hotter zones, in which the temperature is from 800 to 1600 C., to such an extent that the total free cross-sectional area of the passages is not greater than 40% of the total cross-sectional area of the regenerator. In this way, the heat transfer coefficient for the air is increased relatively to the heat transfer coeicient for the gas. Further, the total heat transmitted and the total heating surface per cubic metre are considerably increased relatively to the lsual construction of multi-zone Cowper regenerator. The checkerwork may contain a number of zones, each of which has narrower passages and thinner bricks than the previous one. This type of checkerwork can be used in all kinds of regenerators in which there is no likelihood of any great amount of clogging.

The invention is illustrated by way of example in the annexed drawing.

Fig. 1 is a vertical cross-section through the checker-work, and Fig. 2 is a cross section on the line 2 2 of Fig. 1.

a is a part of the checkerwork of the rst zone of a regenerator, and b is a part of the second zone. 1In the first zone which is heated to temperatures of from 800 to 1600 C. the total cross-sectional area of the passages is less than 40% of the total cross-sectional area of the checkerwork, whilst in the second zone which is heated to temperatures lower than 800 C. the total cross-sectional area of the passages is more than 40% of the total cross-sectional area of the checkerwork.

Checkerwork for multizone regenerators, particularly air heaters with thick stones and wide passages in the hot zones and thinner stones with narrow passages in the colder zones in which the passages in the hot zones have a free crosssectional area aggregating less than said narrower passages and only 40% or less of the total cross-sectional area.

HANS POHL. ALFRED SCHACK. 

